Acquired drug resistance and cross resistance remains a major cause of chemotherapy failure in ovarian cancer. The goal of our studies is to develop clinically feasible techniques to reverse drug resistance based upon mechanisms of drug resistance in relevant model systems of human ovarian cancer. On the basis of our studies which demonstrated that Buthionine Sulfoximine (BSO) potentiated the cytotoxicity of alkylating agents and platinum compounds by lowering cellular glutathione (GSH) levels, we performed a Phase l trial of the combination of BSO plus melphalan (L-PAM) in drug resistant patients. We demonstrated that BSO can be safely administered and lowers GSH levels both in peripheral mononuclear cells and tumor cells. In addition, we have demonstrated that GSH is involved in the DNA repair process which is also enhanced in platinum-resistant ovarian cancer cells. Inhibition of DNA polymerase A with aphidicolin decreases the repair capacity of tumors cells and restores sensitivity in vitro and in vivo to platinum compounds. We plan a Phase I trial of BSO plus carboplatin followed by a Phase II trial of BSO together with L-PAM. We also plan Phase I-Il trials of aphidicolin plus carboplatin. The purpose of these studies is to determine the toxicity and biochemical effects of BSO and aphidicolin given with carboplatin. GSH levels, BSO and aphidicolin pharmacokinetics, and DNA platination will be the major endpoints. We have recently isolated a human cDNA clone coding for gamma- glutamylcysteine synthetase (gamma-GCS) [the rate-limiting enzyme in GSH synthesis] and demonstrated that the gamma-GCS gene is expressed at very high levels in resistant tumor cells. Our pre-clinical studies will focus upon the mechanisms by which GSH is elevated in cisplatin resistant ovarian cancer cells. These studies include introduction into cisplatin- sensitive ovarian cancer cells of an expression vector containing a full length human gamma-GCS cDNA. We will then determine changes in gamma-GCS enzyme activity, GSH levels, and evaluate the effect of altered GSH synthesis on drug sensitivity. These studies will establish the causal relationship between GSH levels and platinum resistance. They also may provide additional targets for future clinical trials.